In an intercom system which is designed for two-way communication between a plurality of stations, it is standard practice to connect all the stations by a single wire pair. Since each station both transmits and receives signals on this same common pair of wires, some local means is necessary to isolate the receiving circuit or load from the transmitting circuit or local source. This has been accomplished in various ways, such as by use of the transformer hybrid circuit commonly used in telephone circuits and by sidetone suppressor circuits which cancel out the local voice signal from the speaker output.
The present invention is an improvement in that it provides a simpler and more reliable circuit which functions as a bilateral current source for both driving all the other receiving stations on the line from the local source (microphone or the like) and also providing an output to the local load (speaker, earphone, or the like) from the signal received on the line from other stations. It presents a high input impedance to the line, allowing a number of stations to be connected to the line. At the same time the circuit provides the two-wire to four-wire conversion by suppressing sidetones in the output to the receiver or local load.